The present invention relates to a hydraulic brake system for automotive vehicles comprising a master cylinder containing at least two working chambers, and at least two brake circuits. A first brake circuit is in constant hydraulic communication with a first working chamber of the master cylinder, while a pressure control valve is inserted in a second brake circuit connected to the second working chamber. The valve is influenced by the pressure in the second brake circuit in a predeterminable fashion.
In Germany patent application No. 29 26 499, a braking pressure control unit for vehicle brake systems in disclosed through which a first brake circuit, preferably that brake circuit allocated to the front-wheel brakes, is in permanent hydraulic communication with a working chamber of a tandem master cylinder. A second brake circuit is allocated to the wheel brakes of the rear axle of the automotive vehicle and is connected to a second working chamber of the tandem master cylinder. The hydraulic pressure in the brake circuit allocated to the rear-wheel brakes is influenced in a predetermined manner. In excess of a predefined pressure in the front-wheel brakes, the pressure supplied to the rear-wheel brakes is reduced so that the pressure prevailing in the rear-wheel brakes will rise less quickly in relation to the front-wheel braking pressure when the brake is applied. Braking pressure control units of this type are meant to prevent overbraking of the rear wheels which usually results in unstable driving conditions.
Anti-lock systems are also known wherein pressure fluid is removed from the wheel brakes and returned to an unpressurized supply reservoir in the presence of critical slip values. Likewise in such brake systems, overbraking of the wheels at the rear axle of an automotive vehicle is precluded to a large extent.
In hydraulic brake systems in which there is provision of both a braking pressure control valve and a braking pressure control by means of pressure fluid removal from the wheel brakes, there is frequently the disadvantage in practical operation that, during brake slip control, the braking pressure variation in the wheel brakes is adversely effected by the braking pressure control unit. The reason for this is that the solenoid valves allocated to the wheel brakes has a relatively high switching frequency which the braking pressure control unit is not able to easily follow, particularly due to the forces due to inertia. Therefore, in operation of the brake slip control apparatus, there may occur a braking pressure variation in the wheel brakes which is not desired. On the other hand, owing to a brake slip control apparatus, the braking pressure in the wheel brakes of the rear axle is controlled so as to reliably preclude overbraking.
Therefore, it is an object of the present invention to provide a hydraulic brake system wherein the braking pressure characteristic curve is not influenced by the braking pressure control unit during brake slip control.